Approach to measuring strain effects using ring oscillators

ABSTRACT

A ring oscillator system for characterizing substrate strain including, a substrate including a through-substrate-via, at least two ring oscillators, wherein a first ring oscillator is closer to the through-substrate-via than a second ring oscillator, and a logic difference circuit that is configured to receive an input from at least the first ring oscillator and the second ring oscillator, and detect a difference between the signal frequency of the first ring oscillator and the signal frequency of the second ring oscillator.

BACKGROUND Technical Field

The present invention relates to an approach to characterizing straineffects using ring oscillators, and more particularly to an apparatusand method of characterizing strain effects introduced bythrough-silicon-vias (TSV) using Vernier ring oscillators.

Description of the Related Art

Transistors on a substrate can experience changes in their environmentthat can effect performance and reliability. The transistors typicallyhave performance characteristics based on their chemical and physicalmakeup. Metal-oxide-semiconductor field-effect transistors (MOSFETs) area type of transistor having a source, drain, gate, and channel. Thedifferent regions of the MOSFET may be p-doped or n-doped.

SUMMARY

A ring oscillator system for characterizing substrate strain including,a substrate including a through-substrate-via, at least two ringoscillators, wherein a first ring oscillator is closer to thethrough-substrate-via than a second ring oscillator, and a logicdifference circuit that is configured to receive an input from at leastthe first ring oscillator and the second ring oscillator, and detect adifference between the signal frequency of the first ring oscillator andthe signal frequency of the second ring oscillator.

A method of measuring strain using ring oscillators including,fabricating a first ring oscillator a first predetermined distance froma through-substrate-via, fabricating a second ring oscillator a secondpredetermined distance from the same through-substrate-via, where thesecond predetermined distance is greater than the first predetermineddistance, and detecting a difference between the signal frequency of thefirst ring oscillator and the signal frequency of the second ringoscillator.

A method of measuring strain using ring oscillators including,fabricating a first ring oscillator a first predetermined distance froma through-substrate-via on a substrate, fabricating a second ringoscillator a second predetermined distance from the samethrough-substrate-via, where the second predetermined distance isgreater than the first predetermined distance, fabricating a logicdifference circuit on the substrate, and detecting a difference betweenthe signal frequency of the first ring oscillator and the signalfrequency of the second ring oscillator, wherein the signal frequency ofthe first ring oscillator is different from the signal frequency of thesecond ring oscillator, and the logic difference circuit is configuredto provide an output signal that is commensurate with strains impartedto the substrate by the through-substrate-via, wherein a value of theoutput signal is based on the difference in the signal frequencies.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a diagram of two ring oscillators arranged parallel to athrough-substrate-via in accordance with an illustrative embodiment;

FIG. 2 is a diagram of two ring oscillators arranged in concentriccircles around a through-substrate-via in accordance with anillustrative embodiment;

FIG. 3 is a diagram of two ring oscillators and a phase comparator forfrequency measurements in accordance with an illustrative embodiment;and

FIG. 4 is a diagram of a phase comparator for beat frequencymeasurements in accordance with an illustrative embodiment.

DETAILED DESCRIPTION

Principles and embodiments of the present disclosure relate generally tousing ring oscillators to measure strain induced changes in transistorsbased on changes in the gate delay of the inverters (i.e., Logical NOTGates) making up the ring oscillators. This provides a simple electricaltest to monitor through-silicon-via induced stresses.

It has been difficult determining the effect of through-silicon-vias(TSVs) on the performance characteristics (e.g., I-V characteristics) oftransistor devices located different distances from the vias.Micro-Raman spectroscopy and X-ray diffraction, which may be used tomeasure strains, are difficult to perform and can be impossible toimplement on an IC product.

The varying proximity of transistors forming, for example, logicdevices, from a TSV can induce differing behaviors in the devices, wherethe distance to a TSV changes the stress and potentially an electricfields in a transistor. The ring oscillators can provide sufficientsensitivity to measure the effects of the TSVs.

The process of forming TSVs can induce axial and radial strain in asubstrate that may be used for integrated circuits (ICs). The strainsmay be introduced by differential thermal expansion of the metalconductor forming/in the via, which may result in cracking of thesubstrate, for example, with a copper via in a silicon wafer. Withoutbeing bound by theory, it is believed that such strains impact thechannel mobility and threshold voltage in transistors surrounding theTSV. The quality and robustness of a process used to form TSVs can bedetermined by measuring the type and degree of the strain effectsutilizing a sensitive circuit with a characterizable response to thestrain effects.

Since transistor threshold voltage and carrier mobility may be skewed bythe proximity of the transistor to a through-silicon-via (TSV), a ringoscillator closer to the TSV may have an oscillation frequency differentfrom a ring oscillator farther away from the same TSV. The test assemblycan be formed directly on the substrate, and may be used to monitor theinduced strains without delayering or breaking the substrate.

The output of each inverter in a ring oscillator changes by a finiteamount of time after the input has changed. The threshold voltage maydecrease for transistors closer to a TSV.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

In one or more embodiments, the ring oscillator may be a Vernier RingOscillator, where the Vernier Ring Oscillator may include thecharacteristics of a Vernier Delay Line (VDL). The VDL characteristicmay be achieved by implementing two separate ring oscillators. Invarious embodiments including two separate oscillator rings, eachinverter stage of each ring oscillator may have a different stage delayas a result of the inverter's proximity to a given TSV, where the stagedelay may be influenced by an experienced strain. The difference ininverter stage delays may result in a difference in the oscillationfrequency of each ring oscillator, where the difference in oscillationfrequency may be proportional to the experienced strain.

A Vernier technique involves a time-to-digital conversion (TDC) of ameasured signal. The TDC may be determined by propagating two transientsignals (S₁ and S₂) offset from each other by an initial delay time,t_(D). Each of the signals, S₁ and S₂, may be propagated through aninverter chain (C₁ and C₂ respectively), including a plurality ofinverters chained in series, referred to as the Vernier Delay Line(VDL). A principle of a VDL lies in the inverter delay in one inverterchain, C₁, being slightly different from the inverter delay in the otherinverter chain, C₂, by an amount t_(R). After each inverter stage, acomparison circuit, for example, a D-latch, may be inserted across theinverter chains C₁ and C₂ and configured to determine which signal fromwhich delay line arrived first. The output from the comparison circuitmay be a digital output (i.e., 1/0), which may be stored, and/orcounted. After a predetermined number of stages (N), the signal beingpropagated through C₁, which may have originally been delayed by t_(D),may be expected to catch up to the signal propagating through C₂, whichmay have stages with a larger stage delay because C₂ may be further fromthe source of the strain. Once the number of stages has been determined,the bit stream (1/0) from the comparison circuit may essentiallyrepresent the time-to-digital conversion of the delay between S₁ and S₂.The TDC may measure the time interval between the initial start pulsesof the two signals, S₁ and S₂, to the time the two signals arrivetogether at the comparison circuit. The TDC may be a high frequencycounter, where the counter increments when the signal from C₁ or C₂arrives. In various embodiments, the TDC may be held at zero until thestart event, counts during the interval, and then stops counting afterthe two signals arrive essentially simultaneously.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element could be termed asecond element without departing from the scope of the present concept.

In various embodiments, a TSV may have a diameter in the range of about6 μm to about 20 μm. An inverter (i.e., NOT gate) fabricated on thesubstrate is typically much smaller than the dimension of the TSV, whichmay be in the range of about 5 μm to about 25 μm, although other sizesare contemplated. A plurality of inventers forming a ring oscillator maybe fabricated a distance from the TSV. In one or more embodiments, thering oscillator may be formed as a ring of an odd number of inventersaround the TSV, where each inverter is about the same distance from theedge of the TSV. In other embodiments, first ring oscillator and thesecond ring oscillator may be fabricated in parallel lines on asubstrate with a TSV. A logic difference circuit may be fabricated onthe substrate in proximity to the first ring oscillator and/or inproximity to the second ring oscillator. The logic difference circuitmay be configured to receive an input from at least the first ringoscillator and the second ring oscillator.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, which is a diagram of tworing oscillators arranged parallel to a through-substrate-via inaccordance with an illustrative embodiment.

In one or more embodiments, a ring oscillator system 100 may include twoor more ring oscillators, where a first ring oscillator 110 and a secondring oscillator 120 maybe positioned at different locations on asubstrate 90. One or more through-silicon-vias 99 may be formed in thesubstrate, where vias 99 may include a conductive metal fill. In variousembodiments, the through-substrate-vias may be formed in a semiconductorsubstrate and have an aluminum, tungsten, or copper fill, or morespecifically, the through-substrate-vias may be formed in a siliconsubstrate and have a copper fill.

In one or more embodiments, the first ring oscillator 110 and the secondring oscillator 120 maybe different distances from the TSV 99. Invarious embodiments, a first ring oscillator 110 may be located adistance, D₁, from the TSV 99, where the distance, D₁, may be measuredfrom a center point of the TSV or an edge of the TSV. In variousembodiments, second ring oscillator 120 may be located a distance, D₂,from the TSV 99, where the distance, D₂, may be measured from a centerpoint of the TSV or an edge of the TSV. In various embodiments,distance, D₂, is greater than distance, D₁, so second ring oscillator120 is farther from the TSV than first ring oscillator 110. A ringoscillator may be far from the TSV 99 such that any difference indistance of the inverter stages is negligible compared to the overalldistance of the inverter stage(s) to the TSV (D₂,>>ΔD₂), where such ringoscillator may be a reference ring oscillator.

Without being bound by theory, during formation of a TSV, which caninclude thermal processing steps, the substrate my experiencethinning/reflow, which may alter the stress state of the substrate andimpart strains. In various instances, the stress may be due to themis-match between the coefficient of thermal expansion for the substrateand the fill material of the via, and may be expressed asϵ=−(α_(Cu)−α_(Si)) ΔT, where ΔT may be from a heat treatment (e.g.,deposition temperature, high temperature annealing). The TSV stresseffect attenuates as 1/r², where ‘r’ is the distance from the source ofthe stress. The resulting strains may alter the electricalcharacteristics of a field effect transistor (FET) channel, for example,the channel carrier mobility and threshold voltage. Transistors formingthe inverters (e.g., a coupled PMOS and NMOS pair) located differentdistances from a stress source may experience differently attenuatedstresses and strains. The difference in strain experienced by thechannels of FET devices at different distances from the strain sourcemay result in different delays.

In one or more embodiments, the ring oscillators 110, 120 may include anodd number (e.g., 3, 5, 7, 9, etc.) of inverters, such that there arethree or more inverters forming the first ring oscillator 110, and threeor more inverters forming the second ring oscillator 120. The first ringoscillator 110 and the second ring oscillator 120 may include the sameodd number of inverters, so both ring oscillators would initially haveessentially the same frequency. Adding more inverters to the ringoscillator can reduce the oscillator frequency.

In various embodiments, the ring oscillators may have differentgeometric arrangements. The ring oscillators may be a linear arrangementof inventers, or a circular arrangement of inventers or other geometricshaped arrangements (e.g., pentagonal, hexagonal, etc.). The geometricarrangements may be adjacent to a TSV or centered around a TSV 99.

In an exemplary embodiments, as illustrated in FIG. 1, a first ringoscillator 110 having a linearly arrangement of ‘n’ inverters 111, 112,114 (I_(A1), I_(A2), . . . I_(An)) may be located adjacent to a TSV 99.A second ring oscillator 120 having a linearly arrangement of ‘n’inverters 121, 122, 124 (I_(A1), I_(A2), . . . I_(An)) may be locatedfurther from TSV 99 than the first ring oscillator 110. In one or moreembodiments, the second ring oscillator 120 may be on the same side ofthe TSV 99 as the first ring oscillator 110. In various otherembodiments, the second ring oscillator 120 may be on the opposite sideof the TSV 99 as the first ring oscillator 110, or on any other side ofthe TSV 99 as long as one of the ring oscillators is farther from theTSV 99 than the other ring oscillator. The linear arrangement of thefirst ring oscillator 110 and the second ring oscillator 120 may formparallel lines.

A distance may be measured from the component of the ring oscillatorclosest to the TSV to the edge of a TSV. In a linear arrangement, one ofthe inverters may be the closest to the TSV, whereas in a circular orpolygonal arrangement all of the inverters may be approximately an equaldistance from the edge or center of the TSV.

In one or more embodiments, the closest point of the first ringoscillator is in the range of about 3 μm to about 7 μm from the closestedge of the through-substrate-via, and the closest point of the secondring oscillator is in the range of about 10 μm to about 100 μm from theclosest edge of the through-substrate-via.

In one or more embodiments, the output 119 of the first ring oscillator110 and the output 129 of the second ring oscillator 120 may me inputtedinto a logic difference circuit 150, where the logic difference circuit150 may be configured to determine a difference between the first ringoscillator output signal and the second ring oscillator output signal.The ring oscillator outputs may be an oscillating signal having afrequency, where the difference in frequencies may be detected and/ormeasured by superimposing the signals and detecting/counting the beatsgenerated, measuring an output signal having a frequency that is basedon the difference between the signal frequency of the first ringoscillator and the signal frequency of the second ring oscillator, orusing a time-to-digital converter.

In various embodiments, the difference circuit output 155 may indicatethe difference between the first ring oscillator frequency and thesecond ring oscillator frequency, where the output 155 from thedifference circuit may be commensurate with the difference in strainexperienced by the inverters of the first ring oscillator 110 and theinverters of the second ring oscillator 120.

FIG. 2 is a diagram of two ring oscillators arranged in concentriccircles around a through-substrate-via in accordance with anillustrative embodiment.

In one or more embodiments, a ring oscillator may fabricated on asubstrate around a TSV 99. In an exemplary embodiments, as illustratedin FIG. 2, a first ring oscillator 210 may include seven (7) inverters(211, 212, 213, 214, 215, 216, and 217) surrounding a TSV 99 in a circlehaving a diameter, D₁, and a second ring oscillator 220 may includeseven (7) inverters (221, 222, 223, 224, 225, 226, and 227) surroundinga TSV in a circle having a diameter, D₂.

In one or more embodiments, the first ring oscillator 110, 210 may be adistance in the range of about 10 μm to about 30 μm from the edge of thethrough-substrate-via, and the second ring oscillator 120, 220 may be adistance in the range of about 50 μm to about 100 μm from the edge ofthe through-substrate-via.

The first ring oscillator 110, 210 and the second ring oscillator 120,220 may include one or more long channel, p-doped channel metal oxidesilicon field effect transistors (MOSFETs), where a long channel MOSFETcan have a channel with a length that is greater than the sum of thesource width and the drain width, so as to be able to ignore edgeeffects. A long channel FET can be more sensitive to stress, therebyexperiencing a greater difference in gate delay between a more highlystressed device and a less stressed device. A long channel PFET may showthe greatest sensitivity, with time and/or threshold voltage, V_(t),decreasing for devices close to a TSV.

In one or more embodiments, a phase comparator 250 may receive signalsfrom the first ring oscillator 210 and the second ring oscillator 220,and determine a difference, for example, in frequency, between the tworeceived signals, where the difference in signals may be commensurate(e.g., proportional) with the difference in strain effecting first ringoscillator 210 and second ring oscillator 220. The ring oscillatorcloser to the source of strain (e.g., the TSV) may have a higherfrequency than the ring oscillator further away from the source of thestrain. The phase comparator 250 may detect the beats generated by thedifference in frequencies, f_(pc)=f_(ro1)−f_(ro2), of the signalsreceived from first ring oscillator 210 and second ring oscillator 220,and count the beats to establish a measurement of the strain produced bythe TSV 99. The induced strain on the ring oscillator may be quantifiedbased on the difference in the two frequencies.

FIG. 3 is a diagram of two ring oscillators and a phase comparator forfrequency measurements in accordance with an illustrative embodiment.

In one or more embodiments, a reference ring oscillator 330 may beincluded to generate a reference frequency having a predeterminedfrequency value that is fed into the phase comparator 350, which isconfigured to measure the difference in frequencies between the stressedring oscillator 310 and the reference ring oscillator 330,f_(pc)=f_(ref)−f_(stress). f_(stress). The reference ring oscillator 330may be far from the stress source(s) to avoid strain influences, whereasthe stressed ring oscillator 310 may be located 20 μm or less from astress source. In various embodiments, the phase comparator may befabricated on the same substrate as the ring oscillators and the TSV.

In one or more embodiments, the ring oscillator(s) may include a NANDgate 370, 380, where the NAND gate can be used to enable or disable theoscillations, since the oscillator may only function when the NAND gateis enabled.

FIG. 4 is a diagram of a phase comparator for beat frequencymeasurements in accordance with an illustrative embodiment.

In one or more embodiments, a ring oscillator 310 experiencing a strainmay generate an output signal that is sampled using a reference ringoscillator 330, where an output signal is produced that is proportionalto the difference in the frequencies. The strain may be determined bycomparing a signal generated by the first ring oscillator to a signalgenerated by the second ring oscillator to obtain an output signalcommensurate with a difference in strains experienced by the first ringoscillator and the second ring oscillator. In one or more embodiments,the phase comparator 350 may be a Gilbert Cell. In various embodiments,cascoding may be employed to improve the performance of the circuits.

In one or more embodiments, a phase comparator 350 may include circuitryforming a stressed ring oscillator detector circuit 510, and a referencering oscillator circuit 560, where the signal from the stressed ringoscillator 310 is multiplied by the output signal from the referencering oscillator 330. In various embodiments, the output signal from oneof the inverters forming a stressed ring oscillator 310 may be input toinput lines 515 and 518 of a stressed ring oscillator detector circuit510, where the signal is applied to the gates of two cross-coupled fieldeffect transistor (FET) pairs 520, 530, 540, 550. An output signal fromone of the inverters forming the reference ring oscillator 330 may beinput to input lines 565 and 568 of a reference ring oscillator detectorcircuit 560, where the signal is applied to the gates of FETs 570, 580.

In various embodiments, when the inverter output from the stressed ringoscillator 310 is high (i.e., ‘1’) the input lines 515, 518 and thegates for field effect transistors (FETs) 520, 530, 540, 550 are alsodriven high. If the input signal from the inverter of the reference ringoscillator 330 is also high (i.e., ‘1’), then the input lines 565, 568are also high. When the input lines 515, 518 are high, and the inputlines 565, 568 are also high, output lines 590, 595 are coupled toground 599, so are low (i.e., 0, grounded). When the input lines 515,518 are high, and the input lines 565, 568 are low, then output lines590, 595 have a signal driven by FETs 520, 530, 540, 550, such that theoutput signal on lines 590, 595 are proportional to difference infrequencies between the input signal on input lines 515 and 518 andinput signal on input lines 565 and 568. The phase comparator canessentially amplify the difference in the phase of the two inputs,coming from 310 and 330 of FIG. 3, where ring oscillator 310 drives theinput 515, 518 and ring oscillator 330 drives the input 565, 568 toprovide an output voltage on 590 and 595 that is proportional to thedifference in the frequencies of the inputs The output from phasecomparator 350 may be measured to determine the strain induced by theTSV.

Having described preferred embodiments of a system and methodcharacterizing strain effects using ring oscillators (which are intendedto be illustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments disclosed which are within the scopeof the invention as outlined by the appended claims. Having thusdescribed aspects of the invention, with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

What is claimed is:
 1. A ring oscillator system for characterizingsubstrate strain, comprising: a substrate including athrough-substrate-via; at least two ring oscillators, wherein a firstring oscillator is closer to the through-substrate-via than a secondring oscillator; and a logic difference circuit that is configured toreceive an input from at least the first ring oscillator and the secondring oscillator, and detect a difference between the signal frequency ofthe first ring oscillator and the signal frequency of the second ringoscillator.
 2. The ring oscillator system of claim 1, wherein each ofthe at least two ring oscillators comprise the same odd number ofinverter devices.
 3. The ring oscillator system of claim 1, wherein thefirst ring oscillator and the second ring oscillator form concentriccircles around the through-substrate-via, where the first ringoscillator is closer to the edge of the through-substrate-via than thesecond ring oscillator.
 4. The ring oscillator system of claim 3,wherein the first ring oscillator is in the range of about 10 μm toabout 30 μm from the edge of the through-substrate-via, and the secondring oscillator is in the range of about 50 μm to about 100 μm from theedge of the through-substrate-via.
 5. The ring oscillator system ofclaim 1, wherein the first ring oscillator and the second ringoscillator are parallel, and the first ring oscillator is closer to theedge of the through-substrate-via than the second ring oscillator. 6.The ring oscillator system of claim 5, wherein the first ring oscillatorand the second ring oscillator include one or more long channel, p-dopedchannel MOSFETs.
 7. The ring oscillator system of claim 6, wherein theclosest point of the first ring oscillator is in the range of about 3 μmto about 7 μm from the closest edge of the through-substrate-via, andthe closest point of the second ring oscillator is in the range of about10 μm to about 100 μm from the closest edge of thethrough-substrate-via.
 8. The ring oscillator system of claim 1, whereinthe logic difference circuit is a phase comparator that is configured toprovide an output signal having a frequency that is based on thedifference between the signal frequency of the first ring oscillator andthe signal frequency of the second ring oscillator.
 9. The ringoscillator system of claim 8, wherein the logic difference circuit is aphase comparator that comprises a third ring oscillator that isconfigured to provide a references signal at a predetermined frequency.10. The ring oscillator system of claim 1, wherein the signal frequencyof the first ring oscillator is different from the signal frequency ofthe second ring oscillator, and the logic difference circuit isconfigured to provide an output signal that is commensurate with strainsimparted to the substrate by the through-substrate-via, wherein a valueof the output signal is based on the difference in the signalfrequencies.
 11. A method of measuring strain using ring oscillators,comprising: fabricating a first ring oscillator a first predetermineddistance from a through-substrate-via; fabricating a second ringoscillator a second predetermined distance from the samethrough-substrate-via, wherein the second predetermined distance isgreater than the first predetermined distance; and detecting adifference between the signal frequency of the first ring oscillator andthe signal frequency of the second ring oscillator.
 12. The method ofclaim 11, wherein fabricating the first ring oscillator involvesfabricating an odd number of inverters connected in series, andfabricating the second ring oscillator involves fabricating the same oddnumber of inverters connected in series as fabricated for the first ringoscillator.
 13. The method of claim 11, wherein the first ringoscillator and the second ring oscillator are fabricated as concentriccircles around the same through-substrate-via, where the first ringoscillator is closer to the edge of the through-substrate-via than thesecond ring oscillator.
 14. The method of claim 13, wherein the firstring oscillator is in the range of about 10 μm and about 30 μm from theedge of the through-substrate-via, and the second ring oscillator is inthe range of about 50 μm to about 100 μm from the edge of thethrough-substrate-via.
 15. The method of claim 11, wherein the firstring oscillator and the second ring oscillator are fabricated inparallel lines, and the first ring oscillator is closer to the edge ofthe through-substrate-via than the second ring oscillator.
 16. Themethod of claim 15, wherein the first ring oscillator and the secondring oscillator include one or more long channel, p-doped channelMOSFETs.
 17. The method of claim 16, wherein the closest point of thefirst ring oscillator is in the range of about 3 μm to about 7 μm fromthe closest edge of the through-substrate-via, and the closest point ofthe second ring oscillator is in the range of about 10 μm to about 100μm from the closest edge of the through-substrate-via.
 18. The method ofclaim 11, which further comprises providing an output signal having afrequency that is based on the difference between the signal frequencyof the first ring oscillator and the signal frequency of the second ringoscillator.
 19. The method of claim 18, wherein comprises a third ringoscillator that is configured to provide a references signal at apredetermined frequency.
 20. A method of measuring strain using ringoscillators, comprising: fabricating a first ring oscillator a firstpredetermined distance from a through-substrate-via on a substrate;fabricating a second ring oscillator a second predetermined distancefrom the same through-substrate-via, wherein the second predetermineddistance is greater than the first predetermined distance; fabricating alogic difference circuit on the substrate; and detecting a differencebetween the signal frequency of the first ring oscillator and the signalfrequency of the second ring oscillator, wherein the signal frequency ofthe first ring oscillator is different from the signal frequency of thesecond ring oscillator, and the logic difference circuit is configuredto provide an output signal that is commensurate with strains impartedto the substrate by the through-substrate-via, wherein a value of theoutput signal is based on the difference in the signal frequencies.